Debian Python Policy
Appendix A - Build Dependencies

Build dependencies for Python dependent packages must be declared for every
Python version that the package is built for. The python-all-dev
should be used when building extensions for any or all Python versions. To
build for a specific version or versions, Build-Depend on
pythonX.Y-dev.

Some applications and pure Python modules may be able to build-depend only on
python or python-all and not require the -dev
packages. Packages that do not require the -dev packages must not build-depend
on them.